Hmm Tricky. I once had a similar task when a local trouble spot (cunningly disguised as a Pub!) changed management and they set to change the clintele' to that of a more family orientated venue

. If I said it was easy then I would be lying, to change a pub, which previously had a live music only policy of Indie and Metal, into a family pub is a challenge not only for the Manager but for any entertainer foolish enough to be within range of a flying beer glass.
It took a long time, but they did change the venue around, and its now a "Millers Kitchen" with little entertainment and a wacky warehouse

- now you can't get anymore family orientated than that!, so it is possible.
I would advise that you ask the management of the venue to add the fact that it is no longer an R&B night to any advertising or posters that they have around, this should advise a percentage that it is not the usual night.
Also make sure that there is a poster clearly advertising your Non R&B night close to the disco, then if people complain then you can point to the poster and say well sorry, but this is how it was billed and you are purely their to provide a service as instructed by the management.
At awkward functions such as this, I simply point out that I am only following the instructions of the client / manager and that I am perfectly willing to change the music policy providing that the person who booked me comes and confirms the decision.
It may not be as bad as you seem to be anticipating, so I wouldn't beat yourself up over it. Just ask them (management) to change the posters etc